Brushless permanent magnet motors are becoming more widely used in a variety of motor applications. Advantages of brushless permanent magnet motors over other types of AC motors include high efficiency, small size, and economy of fabrication. Additionally, in certain specialized applications, such as gyroscopes, the lack of leads to the rotating member and a minimum number of leads to the stationary member of a brushless permanent magnet motor are important.
A typical brushless permanent magnet motor operates by creating a rotating magnetic field with one or more drive-windings which apply a force to a magnetized rotor causing the rotor to rotate. (Alternatively, the magnet can be stationary with rotating windings, but this is less common.) The excitation signal applied to the motor drive-winding must be synchronized with the position of the permanent magnet rotor so that the proper force is applied to the rotor. Most brushless permanent magnet motor drive circuits require the motor to include a transducer to provide a signal representative of the rotor position with respect to the drive-winding, but the rotor position transducer adds to the complexity and expense of the motor. Extra conductors are required for the transducer signals, which can be a significant disadvantage in some applications, such as gyroscope wheel motors. Hall effect devices are frequently used to sense rotor position. Hall effect devices are, however, sensitive to high temperature. Thus, motor operating temperature range is restricted and reliability may be affected if the motor is overheated.
Permanent magnet motor drive circuits have recently been developed that do not require the motor to have a rotor position transducer. These drive circuits can determine the rotor position in a permanent magnet motor by sensing the voltages across and currents through the drive-windings of the motor and deriving the back EMF in the drive windings. From the back EMF the rotor position can be readily determined. See, for example, U.S. Pat. Nos. 4,162,435, 4,169,990, and 4,275,343. These circuits have difficulty, however, in reliably starting a torque-loaded permanent magnet motor from a stopped state. At zero or very low speeds, the back EMF from a permanent magnet motor stator is very small and difficult to accurately sense. Thus, until the present invention, in transducerless, brushless, permanent magnet motors, control of orientation of the stator magnetic field during start-up has been open loop; i.e. the stator field orientation has not been dependent on sensed rotor position. The consequence of this is that prior art drive circuits for transducerless, brushless, permanent magnet motors cannot reliably provide high torque during start-up, and hence cannot reliably start motors in many applications.
One common method for starting up a transducerless permanent magnet motor is to provide a circuit which during start-up causes the stator windings to produce a magnetic field which is independent of the rotor orientation and which initially rotates slowly and then gradually increases in speed. After the rotor has reached a certain speed, the magnitude of the back EMF signal becomes sufficiently large that it may be reliably used to indicate the rotor position. In some known implementations, control of stator orientation is switched abruptly from open loop to rotor (closed loop) control at a threshold speed. In other implementations, the transfer from open loop to closed loop control occurs gradually as the growing amplitude of the back EMF overcomes a bias signal.
In all transducerless brushless permanent magnet motors known prior to the invention only a small fraction of peak torque should be relied upon for a reliable start, so only lightly loaded motors can be reliably started. In addition starting is relatively slow. Also an additional circuit is sometimes necessary to monitor motor operation and provide an indication that the motor has not properly started, and in this circumstance the starting procedure is repeated.